Adds the VMOV instruction to the integrated assembler. Because #<imm> is
complex, and we don't currently have a special ARM class to encode
constants defined by ARM method AdvSIMDExpandImm(), it currently only
accepts (nonnegative) integer32 constants that can fit into 8 bits.

There are no tests for this instruction, since there currently no
callers to this instruction. However, John will be checking in a CL
shortly that will use this instruction.

Add example generator for vector add.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1879463003 .